Electrochromic devices are devices in which a physical/chemical change produced in response to the induced electric field results in a change in the reflective (or transmissive properties) of the device with respect to electromagnetic radiations, e.g., uv, IR, and visible radiation. Such devices, one embodiment being shown as item 10 in FIG. 1, generally comprise a film of electrochromic material 12 and an ion-conductive insulating layer 14 which functions as an electrolyte layer. The film and the electrolyte layer are in surface contact with each other for exchange of ions between the electrochromic film and the electrolyte layer. Two conductive electrode layers, 16 and 18 in FIG. 1, at least one of them being transparent, are disposed on the opposite outer surfaces of the film and the electrolyte layer to provide means for applying a voltage across the combined thickness of the electrochromic film and the electrolyte layer. The electrode layers, 16 and 18 in FIG. 1, are provided on substrates, 20 and 22 of FIG. 1, which substrates may be of a material such as glass. Depending on the ion providing and ion storage capacity of ion conductive layer 16, a counter electrode located between ion conductive layer 14 and electrode layer 18 may be used. The electrodes are provided with external electrical leads 24 and 26 connected to a voltage providing source 28. Application of a voltage of proper polarity across the electrodes causes coloration of the electrochromic layer. By reversing the polarity of the applied voltage, the colored electrochromic layer will be uncolored (bleached). Changing from the bleached state to the colorless state or from the colored state to the bleached is termed "switching". The electrochromic material may be persistent in either its colored state or its non-colored state. By "persistent" is meant the ability of the material to remain, after removal of the electric field, in the absorptive state to which it is changed, as distinguished from a substantially instantaneous reversion to the initial state. The length of time a material is persistent is called its "open circuit memory" or simply "memory". In some embodiments, the coloration can be erased by simply short circuiting the electrodes through an external circuit, there being enough internally stored charge to supply the reversed voltage required to raise the coloration in electrochromic layer. Electrochromic devices of this type have been described for several uses, such as image display, for light filtering, etc. See, e.g., U.S. Pat. Nos. 3,708,220, 4,194,812; 4,278,329; 4,645,308; 4,436,769; 4,500,878; 4,150,879; 4,652,090; 4,505,021; and 4,664,934.
In such devices, the electrochromic film usually comprises an inorganic metal oxide material, most commonly a transition metal oxide, in particular: tungsten oxide. When tungsten oxide is the electrochromic material, the electrolyte layer is adapted to provide a positively charged light cation, preferably, a proton or a lithium ion. The electrolyte layer is generally a liquid electrolyte solution which comprises polymers or copolymers containing acidic groups such as polystyrene sulfonic acid or a solid compound like lithium chloride. The electrolyte layer also may be a gel like polyvinyl butyral-methanol doped with LiCl.
It would be desirable in some situations to have an electrochromic display device which exibits gradations of color intensity in different regions of the device during operation of the device, for example, to provide a monochromatic "picture" based on the same color but different intensities thereof. It would further be desirable to provide an electrochromic device having a gradient band of coloration, e.g., a more intense coloration in a lower region as compared to its upper region. Such a device could find use as walls of an office that could be switched from the colorless state to a graded colored state at will to provide privacy. It could eliminate the combined glass wall/venetian blind combination often used in offices today. Advantageously it would be easier to keep clean. It further will be appreciated that it might be desirable to have an electrochromic device which includes a gradient band in which the upper portion is more intensely colored than the lower region. Such a device might be useful as the windshield of an automobile, for the windows or buildings or for sunglasses. Ballmer in U.S. Pat. No. 4,529,275 teaches that normal eyeglasses can be used as sunglasses by forming such glasses as electrochromic devices using different contact-electrode techniques to color an eyeglass lens more strongly in the upper region than in the lower regions. According to Ballmer doing so would produce so-called automobile driver's sunglasses, in which an instrument panel is more readily observed through the less colored part of the lenses.
It is not well ascertained how Ballmer means to provide the color gradient but according to his teachings it is related to contact with the electrodes. It appears to suggest providing an electrical field gradient to be induced by limiting the surface conductivity of the electrodes. Such a method would be complex and less than desirable. This patent makes no teaching or suggestion of the invention disclosed herein.
Castellion, in U.S. Pat. No. 3,578,843 directed to reflecting electrochromic devices, discloses that the depth of coloration is dependent upon the thickness of the persistent electrochromic layer. He further discloses that, in theory, it would seem that the thicker the layer, the more color centers would be formed upon application of the electric field and therefore deeper coloration could be expected. He concludes that, however, since thin layers could be expected to color more quickly in some cases, the relationship between thickness of the layers and depth of color is not simple. He makes no teaching or suggestion to provide a device having electrochromic material in a thickness gradient in in at least one region in order to provide a device with different intensities of coloration as is done in the present invention.
It would be desirable if a method could be found to provide a color gradient in an electrochromic device in a less complex and thus commercially advantageous way over that suggested by Ballmer. The invention disclosed herein provides an electrochromic device exhibiting a gradient of color in different regions of the device such as a gradient band on a windshield in a feasible way.